A. Field of the Invention
This invention relates to photo responsive arrays and methods of manufacture. More particularly, the invention relates to integrated phototransistor arrays and methods of manufacture.
B. Description of the Prior Art
Photo sensing arrays translate optical information into corresponding electrical signals for processing. Photodiode and phototransistor arrays are available for such translation. Photodiode arrays generate a uniform current proportional to the light flux illuminating the surface of the array. Such arrays, however, are not sensitive to low light levels. Additionally, amplifiers are required for photodiode arrays to increase their electrical signals to a level sufficient for processing. Phototransistor arrays, in contrast, amplify currents generated by light flux illuminating the surface of the array. The amplifying qualities of phototransistor arrays permit them to operate at lower light levels than those for photodiode arrays. Unfortunately, the amplifying advantage of phototransistor arrays is offset by variations of beta or transfer efficiency of carriers flowing through the base region to the collector junction. Beta is sensitive to base width and surface recombination along the emitter periphery which may vary across the array. The variation in beta across the array results in different phototransistor currents for uniform light flux illuminating the array. The mode of operating also affects the performance of the phototransistor array. A direct current (DC) or continuous mode operation, as described in U.S. Pat. No. 3,617,823 issued Nov. 2, 1971, which utilizes the multiplication of the photon-generated carriers, is obviously sensitive to differences in beta across the array. A dynamic or storage mode of operation, as described in the article entitled "Operation of P-N Junction Photodetectors In A Photon Flux Integration Mode", by G. P. Weckler, IEEE Journal Solid State Circuits, Vol. SC-2, pgs 65-73, September 1967, which depends upon the collector base capacitance and not multiplication of photon generated carriers, is less sensitive to variations in beta. However, dynamically operated phototransistor arrays exceeding several hundred elements develop read out problems due to capacitance coupling between sensing lines. Also, dynamically operated phototransistor arrays require complex clocking and readout circuitry. A phototransistor array that is not limited by (1) beta variation, (2) complex clocking or readout circuitry or (3) size due to capacitance coupling will advance the technology of converting optical to electrical information.